Superluminal motion of a relativistic jet in the neutron-star merger GW170817

Abstract

The binary neutron-star merger GW170817 was accompanied by radiation across the electromagnetic spectrum and localized to the galaxy NGC 4993 at a distance of about 41 megaparsecs from Earth. The radio and X-ray afterglows of GW170817 exhibited delayed onset, a gradual increase in the emission with time (proportional to t) to a peak about 150 days after the merger event, followed by a relatively rapid decline. So far, various models have been proposed to explain the afterglow emission, including a choked-jet cocoon and a successful-jet cocoon (also called a structured jet). However, the observational data have remained inconclusive as to whether GW170817 launched a successful relativistic jet. Here we report radio observations using very long-baseline interferometry. We find that the compact radio source associated with GW170817 exhibits superluminal apparent motion between 75 days and 230 days after the merger event. This measurement breaks the degeneracy between the choked- and successful-jet cocoon models and indicates that, although the early-time radio emission was powered by a wide-angle outflow (a cocoon), the late-time emission was most probably dominated by an energetic and narrowly collimated jet (with an opening angle of less than five degrees) and observed from a viewing angle of about 20 degrees. The imaging of a collimated relativistic outflow emerging from GW170817 adds substantial weight to the evidence linking binary neutron-star mergers and short γ-ray bursts.

Fermi GBM Observations of GRB 150101B: A Second Nearby Event with a Short Hard Spike and a Soft Tail

“…When jets plow through dense environments they deposit a fraction of their energy in a hot cocoon (Ramirez-Ruiz et al 2002;Pe'er et al 2006), which may occur in binary neutron star mergers as the ultrarelativistic jet that powers the SGRB pushes through material ejected just before merger (Lazzati et al 2017). GRB 170817A has odd behavior in both prompt and afterglow emission, the origin of which is a matter of some debate (Abbott et al 2017a;Alexander et al 2017Alexander et al , 2018Bromberg et al 2017;Haggard et al 2017;Kasliwal et al 2017;Margutti et al 2017;Mooley et al 2018aMooley et al , 2018bTroja et al 2017Troja et al , 2018aGottlieb et al 2018;Lazzati et al 2018;Lyman et al 2018;Nynka et al 2018;Ruan et al 2018;Veres et al 2018). Possible interpretations include a structured ultrarelativistic jet (e.g., Alexander et al 2018), a jet and cocoon together (e.g., Abbott et al 2017a), or a cocoon model (e.g., Kasliwal et al 2017) where the shock breakout produces the harder peak.…”

Subsequent Nonthermal Emission Due to the Kilonova Ejecta in GW170817

“…In those models, the emission is due to the mildly relativistic ejecta such as a cocoon or a wide jet. The subsequent rising of the radio and X-ray fluxes as far as ∼ 100 days (Mooley et al 2017;Ruan et al 2017) further supports the mildly relativistic scenario, in which the onset time of the radio and X-ray emission can agree with the deceleration time of the ejecta by adjusting the initial velocity even with a high CBM density. Although a model with a highly relativistic jet can still reconcile the rising light curves (Lazzati et al 2017), a very low CBM density (∼ 10 −5 cm −3 ) is required to suppress the emission from the ultrarelativistic jet.…”

“…If the ambient density is higher than ∼ 10 −2 cm −3 , we can expect detections of radio, optical/IR, and X-ray counterparts in a few years. If the early X-ray and radio counterparts are emitted from the shock produced by the additional low-mass ejecta of v ∼ 0.5c-0.8c (see also Mooley et al 2017), such a high-density case can be justified. In this case, the off-axis GRB model with a low CBM density should be reconsidered.…”

“…Therefore, this low-mass additional component other than the kilonova ejecta is a reasonable assumption. Mooley et al (2017) and Ruan et al (2017) reported the continuous rising of the radio and X-ray light curves as far as ∼ 100 days. While Mooley et al (2017) fitted the radio data by a mildly relativistic ejecta with a power law velocity distribution (E(> Γβ) ∝ (Γβ) −5 ), our simple model with a second "slow" component (see Table 2 Fig.…”

“…The early (<20 days) radio and X-ray data are taken from Alexander et al (2017) and Margutti et al (2017), respectively. The late radio and X-ray data are taken from Mooley et al (2017) and Ruan et al (2017), respectively. and dashed lines in Figure 5) also agrees with the observed light curves.…”